Bleaching cellulose pulp



Patented Aug. 1931 a m STATES. P TE vrice JOHN scnunnn, or ASHEVILLE, Non'rn GAROLINA, nsszeNon TO THE cn'AMrIoN FIBRE COMPANY; or CANTON, NORTH CAROLINA, A CORPORATION or OHIO ,IBLEACHING 'cnmiut'osn PULP No Drawing. I

This invention relates to a process of bleaching cellulose pulp involving the use of an oxidizing bleaching agent, and is particularly concerned with such a process adapted to produce from raw chemical pulp, "particularly, raw sulfate or sulfite pulp, a bleached pulp capable of yielding a paper product possessing not only'a high degree of relative whiteness (i. e., high color) but also a high degree of resistance tofold and totear. I

In the bleaching of cellulose pulp a high color is obtained generally only at the sacrifice in the final paper making product either of strength, of yield, or of resistance to fold and to tear, or of all of these. For example,

. it is known that the alpha, or resistant, cellulose content of sulfite pulp may be increased to a value well over 90% by treat- 0 ment with hot caustic alkali in connection with the bleaching process. While the paper product from the bleached pulp thus obtained possesses a high degree both of relative whiteness and of resistance to fold and 5 to tear, the process results he highly disadvantageous material loss of the raw pulp substance; in addition, this procedure is open to the" objections that it is time consuming and requires large amounts of caustic alkali and of steam. Also, it has been attempted to improve the color of kraft or sulfate pulp by treating the same with a solution containing a sulfite or bisulfite prior to the bleaching treatment with anoxidizing bleaching agent.

This procedure, it has been found, is time consuming and fails in eii'ecting a relative whiteness, strength,,or softness in the final paper product obtained fromg this bleached kraft pulp comparable with the whiteness,

40 strength, orsoftness of papers produced from ordinary commercial grades of bleached sulfitepul.

An object of the present invention, therefore, is to provide an economical process dof bleaching rawcellulose pulp whereby to ef- Applicatiom filed June 30, 1928. Serial No. 289,636.

feet the production of a bleached pulp not I adding to the so-treated pulp a suitable acid reducing agent in an amount s'uificient to bring the pulp suspension to an acidity rep resented by a pH value of from 4.0 to 2.0;-

third, removing soluble residues from the pulp suspension by thorough washing, and fourth, treating the thoroughly washed pulp in aqueous suspension with an oxidizing bleaching-agent in such proportions as to produce the desired relative whiteness of the pulp. A final washing operation, with water, may, of course, be carried out. Or, the

bleached -pulp,'-after being treated inv accordance-with operations one to four incluslve as above set out','may be" subjected to the action of an acid reducing agent, followed by athorough washing with water. By fa mildtreatment with an oxidizing bleaching agent I mean a treatment carried out under such conditions, including proportion of bleaching agent to pulp, asnot materially to impair the strength of the'final product. T v

The use of an acid reducing agent, between stages of bleaching with an oxidizing bleach 7 ing agent, under such conditions as to give a degree of acidity of the pulp suspension represented by pH values of from 4.0 to 2.0, greatly-aids in the removal by water washing of'thos'e compounds and materials in the partially bleached pulp which, if not a removed, interfere with orhegativethe efiicacy of a subsequent treatment with the oxidizing bleaching agent. If the pH value of the pulp thoroughly removed; if, on the other hand, the pH value of the pulp is less than 2.0, the

degree of acidity is so highthat the strength of the fiber may be impaired and the desired results therefore lost.

While the invention is not limited to the use of any specific acid reducing agent,it being essential only that the acid reducing agent be of such a character that it may be added to the pulp suspension in such an amount as to give thereto an acidity represented by a pH value of from 4.0 to 2.0, I prefer to employ an aqueous solution of sulfur dioxide. However, any other suitable acid reducing agent,'such as, for example a solution containing stannou s chloride and hydrochloric acid may be used.

It is common knowledge that, when unbleached chemical pulp is subjected to the action of an oxidizing bleaching agent,e. g.,

bleaching powder,in increasing proportions, there is a critical-proportion of bleach to fiber beyond which the action of the bleach results in a loss of fiber strength without niaterially increasing the whiteness. This critical point depends to some extent upon the varying factors of the pulping process by which the pulp is produced.

In the process according to the present invention there is used in the first stageof bleaching an amount of oxidizing bleaching agent which is known to beless than the critical'amount but'suflicientto cause a material bleaching efi'ect.

Thesoluble products associated with the pulp after this first stage of bleaching may be removed therefrom prior to the addition of the ac d 'red'ucingagent; this is not essential,

and in fact there results a greater economy .in those cases whIre the acid reducing 'agent is added to the partially bleached pulp before the washing operation; Thorough washing of, the pulp after treatment with the-acid reducing agent is essential to the successful operation of the process. The removal by washing of. appreciable quantities of colloidal coloring matter makes it possible to complete the bleaching, with a marked reduction in consumption of bleach, and removal of coloring'materials noteasily attackable by the bleach makes itpossible, Without material loss of. strength-of the pulp, to continue the bleaching until the color of the pulp, in the case of soda and sulfate or kraft pulp, is com. parable with that of commercial bleached sulfite, andis superior to that of the usual commercial 'sulfite in the case of sulfite pulp... The'length of time'during which the acid reducing agent reacts-with pulp is animportant element in the successful operation of the process. While I have observed favorable results after one minute of reaction, more favorable results are obtained as the time of reaction is increased'up to thirty to fortyfive minutes.

The acid reducing agent when present in the above stated amount apparently has the property of reducing the positive electric charges upon (1) certain agents,probably mineral catalytic agents,-norn1ally present in the pulp, which agents when allowed to remain in the pulp during the bleaching treatment with the oxidizing bleaching agent result in the destruction of the strength of the fiber, and (2) the colloidal organic matter which is present as a by-product of the primary bleaching treatment. Were theseagents-and by-products allowed to remain in the pulp during the entire period of bleaching, excessive proportions of bleach would be by-products makes it possible easily to remove these materials by water washing. Addition of the acid reducing agent prior to bleaching provides for the removal of atleast a part of the catalysts detrimental to the strength of the pulpbut does not provide for the removal of the by-products-of bleaching, while the addition of acid reducing agent after the bleaching treatment has been concluded does provide for the removal either of catalysts or of by-products only after the same have exerted. their. detrimental effect upon the whiteness and/or strength of the finished ulp.

p Whatever may be the correct explanation of the mechanics of my process it is a fact that by itthere may be produced from pulps obtained by any of the several chemical pulp.- ing processes, bleached pulps which are more nearly white than are the hitherto known bleached pulps, without material loss in sub stance, and without material loss in strength, or in resistance to fold and to tear, of the final paper product.

As illustrating in greater detail in what manner the process according to the present invention may be carried out in connection pulp depend more or less upon the varying factors of the pulping process by which it is produced, it'may be assumed, for example,-

that the unbleached sulfate pulp has a maximum ball mill bursting strength of 142 points per'pound'per ream of 24 36480, and a tear at maximumbursting strength of 3.67

gra.msper pound per ream." Al0% stock 3 suspension of this unbleached sulfate pulp' is treated with a solution of bleaching powder in amount equal to 14% based upon the weights of the dry pulp substance and of the calcium hypochlorite powder containing 35% available chlorine, or 62.5% of the total bleach used in both stages of bleaching.

When the available chlorine has become exi hausted or nearly so the pulp, 'togetheriwith the by-products-ofg bleaching, is treated, v I

1 1. "Proce"ss-for treatingacellulose pulp to without or with an intervening washing op- .eration, with a-solution of sulfur dioxide in an amount sufiicient to-bring thepulpsuspension to an acidity representedby al value of from 4.0 to 2.0. I have found "that a pH value of about 3.2 gives the."best,,and.-

most economical results. For that purpose there is required approximately 1% of sulfur dioxide based .on the'dry weight of the pulp.

Sufficient water is added to the reaction mass to produce a stock suspension of approximately 3%',;which suspension is agitated for fromf30 to-45 minutes and thereafter is thoroughly washed with water to the removal of all soluble matter. A 3%stock suspension of the resulting washed pulp is then bleached with 8.4% of bleaching powder, and washed. The final washed product is characterized by a bursting strength of 137 points per pound per ream,'a tearing strength of 3.15 grams per pound per ream, a folding endurance of 43 double folds per pound per ream, an alpha (or resist-ant) cellulose content (determined on the pulp) of 84.6%, and a degree of relative whiteness comparable with that of commercial bleached sulfite pulp. The pulp product may be brought to a still higher degree of relative whiteness by preceding the final washing by a treatment with an acid reducing agent such I as that hereinbefore described.

A product made from the same unbleached sulfate pulp treated as in the manner above described except that the treatment with sulfur dioxide was omitted and that the second stage of bleaching was effected with 13% of bleaching powder (rather than 8.4%) was characterized by a bursting strength of 115 points per pound per ream, a tearing strength of 2.67 grams per pound per ream, a'folding endurance of 22 double folds per pound per ream, an alpha (or resistant) cellulose content (determined on the pulp) of from 78 to I 81%, and a color which was far below-that of ordinary commercial sulfite pulp.

While in the above example I have indicated certain specific values and have stated certain specific proportions of the several reagents, I would not consider it to be a depa-rture from the scope of this invention to change these proportions within the broad limits hereinbefore mentioned, or to substitute for sulfur dioxide,-the specific acid reducing agent of the example,another acid reducing agent capable of imparting to the pulp suspension an acidity represented by a pH value of from 4.0 to 2.0. It is to be understood, also, that the invention herein described and hereinafter claimed may success-.

to fo ing the pulp to a mild bleaching treatment fully be practiced with the use of any known oxidizing bleachingagent, and that by the expression a hypoclilorite bleaching agent in certain of the appended claims I mean to include also known equivalents thereof.

produce therefrom a 'pap'erfmaking product p g of, high color-1. and a highfdegree of resistance to fold "and" totear which comprises subinent with an oxidizing bleaching agent,add-

.ing to the resulting partiallybleached pulp in aqueous suspension an acidredueing agent In an amount suflicient'to impart thereto an acidity represented byapH value of from jectin'g the'pu lpfto ajrn'ild bleaching treat- 4.0 to 2.0, thoroughlyjwashing the so treated with a hypochlorite bleaching a ent, adding to the resulting partially bleac ed pulp 1n aqueous suspensionan acid-reducing agent in an amount'sufiicient toimpart thereto an acidity represented by a pH value of from 4.0 to 2.0, thoroughly .washing the so treated pul and thereafter bleaching-the washed pulp with an additional amount of a hypochlorite bleaching agent.

3. Process for treating a cellulose pulp to produce therefrom a paper making productv of high color and a high degree of resistance to fold and to tear which comprises subjecting the pulp to a mild bleaching treatment with an oxidizing bleaching agent, adding to the resulting partially bleached pulp in aqueous suspension a solution of sulfur dioxide in an amount suflicient to impart there a to an acidity represented by a pH value of from 4.0 to 2.0, thoroughly washing the so treated pulp, and thereafter bleaching the washedpulp with an additional amount of an oxidizing bleaching agent. I

4. Process for treating a cellulose pulp to produce therefrom a paper making product of high color and a high degree of resistance to fold and to tear which comprises subjecting the pulp to a mild bleaching treatment with an oxidizing bleaching agent, adding to the resulting partially bleached pulp in aqueous suspension an acid reducing agent in an amount sufficient to impart thereto an acidityrepresented by a pH value of from 4.0 to 2.0, thoroughly washing the so treated pulp, thereafter bleaching the washedpulp with an additional amount of oxidizing bleaching agent, treating the bleached pulp with an acid reducing agent in amount sufficient to impart thereto an" acidity represented by a pH value of from 4.0 to 2.0, and finiilly thoroughly Washing the so treated pu p.

5. Process of bleaching cellulose pulp by means of an oxidizing bleaching agent in one or more stages characterized by a treatment of the pulp after one of said stages in aqueous suspension With an acid reducing agent in amount sufficient to impart to the pulp suspension an acidity represented by a pH value of from 4.0 to 2.0.

6. Process of bleaching cellulose pulp as defined in claim 5, in which the treatment with acid reducing agent follows a mild treatment with an oxidizing bleaching agent. In testimony whereof, I afiix my signature.

JOHN SGHUBER. 

